The increasing size, density, and complexity of random access memories (RAMs) makes it extremely difficult to fabricate a RAM that is entirely free of defects. RAM defects can be reduced by improved fabrication methods and increased process controls but such approaches have practical limitations. Overly complex fabrication methods can increase costs due to specialized equipment and materials. Process controls cannot entirely eliminate defects because of the many uncontrollable elements involved in fabricating semiconductor devices.
An alternate approach to increasing the yield of a semiconductor memory device is by circuit designs that can compensate for manufacturing defects. One such approach is the use of redundancy schemes in RAMs. RAM redundancy schemes are well known in the art, and typically involve manufacturing a number of redundant memory cells in addition to the "standard" memory cells. The device is tested after fabrication and if defective standard cells are detected, they are replaced by the redundant cells.
Commonly-owned U.S. Pat. No. 5,377,146 entitled HIERARCHICAL REDUNDANCY SCHEME FOR HIGH DENSITY MONOLITHIC MEMORIES and filed on Jul. 23, 1993, discloses a redundancy scheme wherein a RAM includes standard blocks of memory cells with redundant columns and rows, as well as redundant blocks of memory having their own built-in redundancy. Additional redundancy is provided by subdividing the redundant blocks into redundant groups of rows and columns.
Implementing redundancy schemes requires two general steps. First, defective cells must be disabled. Second, the replacement redundant cells must be enabled. While a number of methods for implementing redundancy schemes exist in the prior art, such as "antifuses," non-volatile memories, programmable logic, and current fuses, these methods involve either complex processing steps, complex additional circuitry, or lack reliability. For simplicity and reliability, laser fusible links remain a popular choice. As is well known in the art, laser fusible links are typically composed of polysilicon and covered by a uniform layer of dielectric, such as silicon dioxide. A laser is employed to evaporate a portion of the link to create an electrical open.
Prior art redundancy schemes employing laser fusible links are known to use two sets of fusible links. The first set of links are "disable" fuses which are used to disable a row and/or column of standard cells. The second set of fusible links are "enable" fuses, and are used to enable a row and/or a column of redundant cells to replace the disabled standard cells. These two sets of fuses are provided in two different "fuse banks". Each fuse bank is an alignment of its respective fuses, usually with a minimum pitch and uniform dielectric covering. Sufficient area is provided around the fuse banks to ensure that no peripheral devices or interconnects can be damaged by the laser during repair. In order to reduce the amount of logic or interconnect necessary to implement such a redundancy scheme the fuses are often integrated into critical decoder or row/column driver paths. A drawback to this type of arrangement is that the fuses must be situated proximate their respective rows or columns. While there may be sufficient area for a bank of disable fuses adjacent to an array, a bank of enable fuses must occupy areas in the central portion of the die that are critically needed for decoder and interconnect circuits. To address this problem it is known in the prior art to position fuse banks on the periphery of the die. While providing more central area on the die, such an arrangement requires an additional interconnect scheme to integrate the fuses with the remainder of the redundancy circuits. This adds size and complexity to the redundancy scheme.
A second type of prior art redundancy scheme utilizes only one bank of fuses for a given set of columns or rows. Like the first type of scheme described above, a selected fuse or a selection of fuses are opened to disable a defective row or column. Unlike the previous redundancy scheme, the opened fuse(s) automatically enables a replacement redundant row or column. While requiring fewer fuses to implement, this second scheme requires more logic and can be less flexible than the two fuse bank redundancy scheme mentioned above.
Another drawback of prior art redundancy schemes is that the fuse layouts are not always conducive to rapid repair. It is known in the prior art to orient a "column" fuse bank perpendicular to the columns of an array, and a "row" fuse bank perpendicular to the rows of the array. As a result, the fuse banks for row redundancy are perpendicular to fuse banks for column redundancy, requiring the laser repair apparatus to step across two directions. In the event the device includes multiple arrays, a number of multiple passes in differing directions are required to effectuate laser repair.
It is always desirable to provide a redundancy scheme of reduced die area. It is also desirable to provide a redundancy scheme that overcomes the drawbacks of the prior art by providing a redundancy scheme employing laser fusible links that can be quickly implemented without requiring undue interconnect arrangements and/or additional logic.